(Background Art) Electromagnetic interference (EMI) has increased in recent years with the advances in the performance of electrical and electronic equipment and the growing use of such equipment. A variety of displays are the cause of EMI as well. For example, a PDP is an assembly composed of a glass plate having a data electrode and a fluorescent layer and a glass plate having a transparent electrode, and generates a large amount of electromagnetic waves when it is in operation, so that it is necessary to shield these electromagnetic waves. The required efficiency in shielding electromagnetic waves with frequencies of 30 MHz to 1 GHz that are emitted from the front of a PDP is 30 dB or more. Electromagnetic wave noises are roughly classified into conductive noise and emissive noise. To reduce conductive noise, a noise filter or the like is commonly used. On the other hand, since electromagnetic insulation of space is needed to cut emissive noise, a metal is used to produce the body of the display, a metal plate is placed between the circuit bases, or the cables are wrapped in metal foil. These measures are effective in shielding electromagnetic waves emitted from the circuits or power blocks, but not effective in cutting electromagnetic waves emitted from the screens of such displays as CRTs and PDPs. Moreover, to cover the screens with metal plates is not appropriate because metal plates are not transparent.
Accordingly, for shielding of electromagnetic waves emitted from display screens, there have been proposed a variety of electromagnetic wave shielding materials (electromagnetic wave shielding devices) that can shield electromagnetic waves with frequencies in the range of megahertz to gigahertz, and, at the same time, are transparent to electromagnetic waves with frequencies in the visible light range. Such electromagnetic wave shielding materials are now commercially available. Of these electromagnetic wave shielding materials, the most typical one is an electromagnetic wave shielding material composed of a transparent substrate made of a resin sheet and a mesh (network or grating) made from a metallic electrical conductor, laminated to the transparent substrate. In electromagnetic wave shielding materials of this type, recently demanded is an electromagnetic wave shielding material having such a structure as is shown in FIG. 4, in which irregularities in the metal mesh surface are smoothed by further applying a transparent resin to the metal mesh and filling the openings of the metal mesh with the resin.
The recent displays, especially PDPs, are characterized by having large-sized screens. The sizes (external dimensions) of electromagnetic wave shielding materials for use as front panels for such displays are as large as 621×831 mm for 37-inch displays and 983×583 mm for 42-inch displays, for example, and there exist electromagnetic wave shielding materials larger in size than these ones. It has been found that, in the whole process ranging from the production of an electromagnetic wave shielding sheet comprising a metal mesh and a transparent resin layer formed on the metal mesh, to the incorporation of the electromagnetic wave shielding material in a display, and also for a long duration of service, the transparent resin layer can lift from or can be separated from the metal mesh because of the large size of the electromagnetic wave shielding sheet.
As shown in FIG. 4, a transparent resin layer 17 has to fully cover a mesh portion 103 that will be faced to a display screen 100. It is necessary to make the area of coating of the transparent resin layer 17 greater than the area of the mesh portion 103 so that the mesh portion 103 is fully covered with the transparent resin layer 17 even if the position of application of a transparent resin varies (positional errors are caused). Further, the transparent resin applied flows and spreads towards the periphery before it is solidified. Practically, therefore, the transparent resin layer is formed so that it covers the entire mesh portion 103 and a 2-3-mm wide, mesh portion-surrounding portion (portion B) of a frame area (a metal layer having no openings) 101 to be used for grounding. In the mesh portion 103, it is easy for the transparent resin layer 17 and the metal mesh 103 to firmly adhere to each other because of the anchor effect taking place between the transparent resin layer 17 and the metal mesh 103 and of the chemical adhesion of the transparent resin layer 17 to an adhesive layer 13. However, in the frame area 101, the transparent resin layer 17 is in contact only with the metal layer that is smooth, so that neither the anchor effect nor the chemical adhesion to the adhesive layer 13 can be anticipated. Moreover, the portion B is the end of an interface between the transparent resin layer 17 and the electromagnetic wave shielding layer (metal layer) 15, so that stress concentrates in this portion. It is therefore considered that the separation of the transparent resin layer 17 from the metal mesh 103 easily occurs in this portion.
Accordingly, in addition to excellent electromagnetic wave shielding properties and moderate transparency (visible light transmittance), a requirement that constituent layers do not cause lifting or separation in the course of production and for the duration of service has come to be newly imposed to those electromagnetic wave shielding materials for displays that use metal meshes.
(Prior Art) Heretofore, there has been known an electromagnetic wave shielding material comprising a transparent plastic substrate and a mesh portion formed on the transparent plastic substrate by the use of an electrically conductive material such as a metal, in which irregularities of the mesh surface are smoothed by partly or entirely covering the mesh portion with a transparent resin layer (see Patent Documents 1 and 2, for example).
The above inventions are aimed at preventing, by filling the openings of the mesh with the transparent resin to smooth the mesh surface, irregular reflection of light that is caused by air bubbles remaining after such a layer as an antireflection filter has been laminated to the mesh surface with an adhesive layer, and also enhancing transparency by smoothing the roughened surface of the adhesive layer exposed at the openings. However, in an attempt to carry out these inventions to make the electromagnetic wave shielding material, we encountered another problem that should be solved. An electromagnetic wave shielding material for a display screen usually has, around a mesh portion, a metallic frame area 101 for grounding, having no openings. A transparent resin layer 17 with which the entire surface of the mesh portion 103 should be coated is formed on an area larger than the mesh portion 103 in order to ensure that the mesh portion 103 is fully covered with the transparent resin layer 17 even if the position of application of a transparent resin varies, and, in addition, the transparent resin applied flows and spreads, so that the end portion B of the transparent resin layer 17 comes in the frame area 101. In the frame area 101, the transparent resin layer 17 is in contact with a flat, smooth metal surface, so that the adhesion between the transparent resin layer 17 and the frame area 101 is inherently weaker than the adhesion between the transparent resin layer and the mesh portion. Moreover, external forces are exerted on the end portion B of the transparent resin layer, and peel stress thus concentrates in this portion. The newly raised problem is that the separation of the transparent resin layer 17 from the frame area 101 frequently occurs in the end portion B for the above-described reason. The above-described patent documents neither describe nor suggest this problem, that is, the lifting or separation of constituent layers of an electromagnetic wave shielding material, and a means of solving the problem.
Patent Document 1: Japanese Patent No. 3570420.
Patent Document 2: Japanese Laid-Open Patent Publication No. 311843/2002.